Experiments that support an isotropic orbital symmetry of the superconducting order parameter in the bulk of Bi2Sr2CaCu2O8+δ

Abstract: Although most workers in the field of high-temperature superconductivity have long believed that the cuprates have a superconducting order parameter that have predominantly dx2-y2 orbital symmetry, the experiments supporting this notion were either surface based, where oxygen loss is severe, or could not distinguish between a superconducting and a non-superconducting, such as a charge-density wave or "pseudogap" order parameter, especially when these two orderings are very inhomogeneous. However, there are now three experiments that strongly suggest that the bulk orbital symmetry of the superconducting order parameter in Bi2212 is isotropic within the CuO2 planes. These are: the1999 c-axis twist experiment of Li Qiang et al.(along with the cross-whisker experiments by Y.Takano and Yu.Latyshev, which lead to the same conclusion), the 2013 Wang Eryin et al.(Tsinghua University) ARPES proximity experiment between Bi2212 and Bi2Se3, and the more recent Zhong Yong et al.(also Tsinghua University) local STM proximity experiment between Bi2212 and monolayer CuO2.The first two of these experiments involved Josephson tunneling across the internal junctions in Bi2212, which are far from the crystal edges where oxygen loss is severe. In the third experiment, the surface was protected by the CuO2 monolayer after cleavage of the Bi2212 single crystal in high vacuum at low temperature and molecular beam epitaxial CuO2 deposition. In addition, the1999 Mößle-Kleiner c-axis Josephson tunneling experiment between Bi2212 and Pb showed at least a very small amount of s-wave order parameter in Bi2212. In addition, the observation (Kashiwagi et al., 2015) of a narrow linewidth emission at 2.4 THz from the internal Josephson junctions of thermally-managed Bi2212 disk devices in a copper holder provide clear evidence that the minimum gap value must exceed 9.8 meV. Although a dx2-y2+idxy order parameter (with equal components, as proposed by Tony Leggett) could explain most of these experiments, such a scenario appears difficult to reconcile with the Mößle-Kleiner and ZhongYong experiments, and with the observation (Tsujimoto et al.,2015) of terahertz emission from Pb-doped Bi2212.